Hybrid receiver for concentrated photovoltaic-thermal power systems, and associated methods

ABSTRACT

A hybrid receiver for a concentrator photovoltaic-thermal power system combines a concentrator photovoltaic (CPV) module and a thermal module that converts concentrated sunlight into electrical energy and thermal heat. Heat transfer fluid flowing through a cooling block removes waste heat generated by photovoltaic cells in the CPV module. The heat transfer fluid then flows through a helical tube illuminated by sunlight that misses the CPV module. Only one fluid system is used to both remove the photovoltaic-cell waste heat and capture high-temperature thermal energy from sunlight. Fluid leaving the hybrid receiver can have a temperature greater than 200° C., and therefore may be used as a source of process heat for a variety of commercial and industrial applications. The hybrid receiver can maintain the photovoltaic cells at temperatures below 110° C. while achieving overall energy conversion efficiencies exceeding 80%.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional patent applicationNo. 62/861,716, titled “Concentrated Solar Photovoltaic and PhotothermalSunflower Receiver and System” and filed on Jun. 14, 2019, the entiretyof which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with U.S. Government support under grant numberARPA-E DE-AR0000473 from the U.S. Department of Energy. The UnitedStates government has certain rights in the invention.

BACKGROUND

Fossil fuels, including coal, oil, and natural gas, are currently theworld's primary energy source. Formed from organic material over thecourse of millions of years, fossil fuels are finite resourcescategorized as non-renewable energy resources. According to the U.S.Energy Information Administration, the burning of fossil fuels wasresponsible for 76% of U.S. greenhouse-gas emission in 2017. These gasescontribute to the greenhouse effect and could lead to potentiallycatastrophic changes in the Earth's climate. Problems with fossil fuelsare related not only to global warming, but also to such environmentalconcerns as air pollutions, acid precipitation, and ozone depletion.

Renewable energy sources and technologies provide for sustainable energydevelopment and avoid the impending shortage of fossil fuels. Renewableenergy is derived from resources that are replenished naturally on ahuman timescale. Such resources include biomass, geothermal heat,sunlight, water, and wind. All of these sources are essential componentsof a nation's energy strategy because of concerns not only for the localand global greenhouse gas emissions, but also for energy security andsustainability. The potential for renewable sources is enormous as they,in theory, can produce many times the world's total energy demand. Inthe past thirty years, a variety of renewable energy technologies andenergy efficiency measures has led to overall cost savings, making thedisplacement of fossil fuels possible with minimal increase in cost.Among these technologies, solar energy is a promising renewable energyresource that can be utilized in many places throughout the world.

SUMMARY

Solar energy can be converted into electrical energy through thephotovoltaic (PV) effect. Semiconductor materials, such asmonocrystalline silicon, polycrystalline silicon, microcrystallinesilicon, copper indium selenide, cadmium telluride, gallium arsenide,and others, are used commercially to produce PV cells that are combinedinto PV panels and modules. One way to boost photoelectric energyproduction is to use optics that concentrate solar power onto a PV cellor module. These concentrator photovoltaic (CPV) modules arecost-competitive when used with high-efficiency multijunction GaAs-basedPV cells. Energy conversion efficiency, which quantifies the portion ofsunlight energy that is converted into electrical energy, varies from10.2% for amorphous silicon-based PV cells at a concentration of 1 sunto 46.9% for III-V multijunction PV cells at a concentration of 508suns.

Solar energy can also be collected as thermal energy. Thermal receiversare devices that absorb solar radiation, converting it into heat andthen transferring the heat to a fluid such as air, water, or oil. Solarreceivers can be classified as non-concentrating or concentrating basedon whether concentrating optics are used or not. A concentrating thermalreceiver typically works with a parabolic mirror or Fresnel lens thatfocuses sunlight onto the thermal receiver, thereby achieving the hightemperatures needed for industrial applications and electric powerproduction.

To more efficiently use solar energy, hybrid concentratorphotovoltaic-thermal (CPVT) power systems have been developed thatcombine CPV modules with thermal receivers to generate electrical energyand thermal energy simultaneously. Industrial process heat accounts formore than two-thirds of the world's total industrial energy consumption,which is a large market for solar energy that is almost entirelyuntapped. Most of the demand is for temperatures below 250° C. CPVTpower systems can reach these temperatures due to the separateconfigurability of CPV modules and thermal receivers. Applicationsinclude food, wine and beverages, textiles, machinery, solar heating,desalination, enhanced oil recovery, and wood pulp and paper processing.

Although III-V multi junction PV cells have demonstrated improvedperformance compared to single-junction PV cells, more than half of theabsorbed solar energy is converted to thermal energy, causing thejunction temperature to rise. Since the efficiency of a PV celltypically decreases as its temperature increases, cooling systems arefrequently used to keep PV cell efficiency optimized. Many activecooling systems for CPV modules pump a heat transfer fluid or gas acrossa thermally conductive backplane upon which the PV cells are mounted.The waste heat is captured by the fluid (e.g., via thermal conductionfrom the backplane into the fluid) and carried away from the PV cells.This waste heat may be dumped, which reduces system efficiency.Alternatively, the waste heat can be utilized for low-temperature (e.g.,less than 80° C.) process-heat applications.

The present embodiments feature a hybrid receiver for CPVT power systemsthat combines a CPV module with a thermal module that acts as both athermal receiver and cooling system for PV cells in the CPV module. Thehybrid receiver uses the same fluid for both PV-cell cooling and thermalpower generation, advantageously increasing efficiency by contributingthe waste heat from the PV cells to the generated thermal power.Therefore, instead of dumping the waste heat from the PV cells, oroutputting the waste heat via a low-temperature thermal output, thepresent embodiments feature only one high-temperature thermal output ata temperature that is higher than what the thermal receiver couldgenerate on its own.

The hybrid receiver may be placed between a concentrator mirror and itsfocal plane, where rays of concentrated sunlight from the concentratormirror are not parallel. Some of the concentrated sunlight directlyilluminates the CPV module, where it is converted to electrical power.Some of the concentrated sunlight also direct illuminates the top andsides of the thermal module, where it is absorbed as thermal energy. Insome embodiments, the hybrid receiver also includes a reflective shroudthat surrounds the hybrid receiver, and reflects scattered light fromthe hybrid receiver back onto to the thermal module, where it can beabsorbed. The shroud also reduces convective energy loss of the thermalmodule by blocking wind currents. Heat transfer fluid flowing underneaththe CPV module carries away waste heat from the PV cells. The heattransfer fluid then flows through a helical tube to absorb heat fromboth direct sunlight and scattered light reflected onto the helical tubeby the shroud. The heat transfer fluid is continuously heated as itflows through the helical tube. The heat transfer fluid then exits thehelical tube, where it can be used for high-temperature process-heatapplications. The heat transfer fluid can be water, oil, or anotherfluid used for cooling and process heat.

With the present embodiments, the ratio of electrical power to thermalpower can be easily changed by moving the hybrid receiver closer to, oraway from, the focal plane of the concentrator mirror. This movementchanges what fraction of concentrated sunlight reaches the CPV moduleversus the helical tube. With water as the heat transfer fluid, thepresent embodiments can maintain PV-cell temperatures below 110° C. withoutput temperatures of the heat transfer fluid above 200° C.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 are a perspective view and side view, respectively, of aconcentrator photovoltaic-thermal (CPVT) power system with a hybridreceiver, in an embodiment.

FIG. 3 illustrates fluid cooling of photovoltaic (PV) cells in anoptoelectronic stack.

FIG. 4 illustrates how the present embodiments implement fluid coolingof an optoelectronic stack, in embodiments.

FIG. 5 is a perspective view of a hybrid receiver that advantageouslyuses only one heat-transfer-fluid system to both cool PV cells andabsorb thermal heat from sunlight, in an embodiment.

FIGS. 6, 7, and 8 are a perspective view, side view, and bottom view,respectively, of CPV and thermal modules of the hybrid receiver of FIG.5, in an embodiment.

FIG. 9 is a side cut-away view of the CPV module of FIG. 5 in regionswhere a PV cell is present, in an embodiment.

FIG. 10 is a top view of the CPV module of FIG. 5 showing the layout ofPV cells, in an embodiment.

FIG. 11 shows exemplary current-voltage curves measured with atriple-junction PV cell.

FIG. 12 shows exemplary temperature contour plots for a prototype CPVmodule, cooling block, and helical tube.

FIG. 13 shows exemplary current-voltage curves measured with a “bare”triple-junction PV cell and each of four quadrants of the prototype CPVmodule.

FIG. 14 shows an exemplary setup and results for tests of the prototypecooling block.

FIG. 15 shows an exemplary setup and results for tests of the prototypehelical tube.

FIG. 16 shows exemplary plots of a convective heat transfer coefficienth as a function of mass flow rate for the cooling block and the helicaltube.

FIG. 17 shows an exemplary plot of pressure drop as a function ofmass-flow rate.

FIG. 18 shows an exemplary flux map measured for a dish and tracker thatmay be used to test a prototype of the hybrid receiver of FIG. 5.

FIG. 19 shows an exemplary plot of solar power incident on the CPVmodule and the helical tube as a function of cell-plane distance inboardfrom the focal plane of the dish.

FIG. 20 shows exemplary plots of predicted performance of the prototypehybrid receiver.

FIG. 21 is a table listing predicted electrical and thermal energiesgenerated by the prototype hybrid receiver when operating at differentdistances between the CPV module and a focal plane of the dish.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 and 2 are a perspective view and side view, respectively, of aconcentrator photovoltaic-thermal (CPVT) power system 100 that uses ahybrid receiver 110 to convert sunlight 106 into electrical power andthermal power. The CPVT power system 100 includes a concentrator dish102 that reflects and concentrates sunlight 106 onto the hybrid receiver110 as concentrated light 108. Some of the concentrated light 108 isconverted into electrical energy by a concentrator photovoltaic (CPV)module of the hybrid receiver 110 (e.g., see CPV module 504 in FIG. 5).Some of the concentrated light 108 is converted into thermal heat by athermal module of the hybrid receiver 110 (see thermal module 502 inFIG. 5). The hybrid receiver 110 is located in front of the focal planeof the dish 102 (i.e., between the dish 102 and the focal plane), andoriented to directly face the dish 102. The hybrid receiver 110 may beused at low concentrations (i.e., less than 100 suns), mediumconcentrations (i.e., up to 300 suns), or high concentrations (i.e., upto 1000 suns, or more).

The CPVT power system 100 also includes a two-axis solar tracker 104that changes an elevation angle, an azimuthal angle, or both, of theconcentrator dish 102 as the sun moves across the sky. The hybridreceiver 110 is affixed to a support arm 112 which, in turn, is affixedto the solar tracker 104. Therefore, as the dish 102 moves, the hybridreceiver 110 is always located at the same distance from of the dish 102and oriented facing the dish 102. Tracking of the sun with the two-axissolar tracker 104 maximizes the amount of the sunlight 106 that the dish102 focuses onto the hybrid receiver 110, thereby maximizing theelectrical power and thermal power generated by the hybrid receiver 110.While the dish 102 is shown in FIGS. 1 and 2 as a parabolic dish, theconcentrator 102 may be another type of concentrator mirror or dishwithout departing from the scope hereof.

FIG. 3 illustrates fluid cooling of photovoltaic (PV) cells 306 in anoptoelectronic stack 300. The optoelectronic stack 300 may be used asthe CPV module of the hybrid receiver 110 of FIGS. 1 and 2. Concentratedlight 108 passes through a transparent superstrate 304 to illuminate thePV cells 306, which convert some of the concentrated light 108 intoelectrical power. Waste heat 312 from the PV cells 306 is conductedthrough a heat transfer plate 308 and into a cooling block 310 throughwhich heat transfer fluid 320 flows. A heat exchanger 322 removes theheat 312 from the heat transfer fluid 320, and pumps the heat transferfluid 320 so that it flows with a sufficient speed to cool the coolingblock 310, and therefore the heat transfer plate 308 and photovoltaiccells 306.

FIG. 4 illustrates how the present embodiments implement fluid coolingof an optoelectronic stack 400, in embodiments. Like the optoelectronicstack 300 of FIG. 3, the optoelectronic stack 400 contains PV cells 306that generate waste heat 312. In FIG. 4, heat transfer fluid 320, afterabsorbing waste heat 312, flows along a helical path 404 located behindthe optoelectronic stack in the z direction (see right-handed coordinatesystem 420). The optoelectronic stack 400 faces the dish 102 in the +zdirection to receive some concentrated light 108. In the x-y plane, theoptoelectronic stack 400 is centered on an optical axis 440 of the dish102. In the z direction, the optoelectronic stack 300 is located“inboard” of a focal plane 442 of the dish 102, i.e., between the dish102 and the focal plane 442.

Centered light 430 of the concentrated light 108 propagates along ornear the optical axis 440. More specifically, and as shown in FIG. 4,rays of centered light 430 are nearly parallel to the optical axis 440(i.e., angles between the rays of centered light 430 and the opticalaxis 440 are near zero). By contrast, off-centered light 432 propagatesfarther from the optical axis 440. More specifically, and also shown inFIG. 4, rays of off-centered light 432 are displaced in the x-y planefarther from the optical axis 440 than centered light 430, and formlarger angles with respect to the optical axis 440 than centered light430. Due to these larger displacements and angles, off-centered light432 misses the optoelectronic stack 400 and instead illuminates sides ofthe helical path 404, where the heat transfer fluid 320 absorbs thethermal energy of the off-centered light 432. Note that off-centeredlight 432 converges since the optoelectronic stack 300 is located“inboard” of a focal plane 442. If the optoelectronic stack 300 werelocated on the other side of the focal plane 442, off-centered light 432would diverge, thereby missing the sides of the helical path 404.

Advantageously, the fluid cooling shown in FIG. 4 increases the thermalenergy generated by the thermal module by utilizing the waste heat 312from the CPV module. The heat transfer fluid 320 can therefore achieve ahigher temperature, when it exits the thermal module, than the maximumsolar cell temperature.

FIG. 5 is a perspective view of a hybrid receiver 500 thatadvantageously uses only one heat-transfer-fluid system to both cool PVcells and absorb thermal heat from sunlight. The hybrid receiver 500 isone example of the hybrid receiver 110 of FIGS. 1 and 2. The hybridreceiver 500 includes a CPV module 504 that uses PV cells 306 to convertconcentrated light 108 into electrical power. The hybrid receiver 500also includes a thermal module 502 beneath the CPV module 504 thatabsorbs additional concentrated light 108 to generate thermal power. Toenhance thermal power generation in some embodiments, the hybridreceiver 500 may include a reflective shroud 506 surrounding the thermalmodule 502 and CPV module 504. The shroud 506 acts as a secondaryreflector that redirects indirect light 508 from the CPV module 504 andthermal module 502 back toward the thermal module 502 so that it can beabsorbed. Indirect light 508 may include: (a) concentrated light 108that scatters off of the CPV module 504 and thermal module 502 (e.g.,via reflection or diffraction), and (b) thermal radiation emitted by theCPV module 504 and thermal module 502. Although not shown in FIG. 5,indirect light 508 propagates away from the CPV module 504 and thermalmodule 502 in all directions. In embodiments, and as shown in FIG. 5,the shroud 506 encircles the thermal module 502 without gaps to maximizethe amount of indirect light 508 redirected back to the thermal module502. The shroud 506 also minimizes convective energy loss of the thermalmodule 502 by blocking wind currents.

FIGS. 6, 7, and 8 are a perspective view, side view, and bottom view,respectively, of the CPV module 504 and thermal module 502. FIG. 7 alsoillustrates how the CPV module 504 receives centered light 430 and howthe thermal module 502 receives non-centered light 432. FIGS. 6, 7, and8 are best viewed together in the following description.

The thermal module 502 includes a helical tube 602 and a cooling block604. The CPV module 504 includes an array of PV cells 306 mounted on aheat transfer plate (see heat transfer plate 920 in FIG. 9) that is indirect thermal contact with a top face 606 of the cooling block 604. Asshown in FIGS. 5-8, the cooling block 604 may be shaped such that theentire bottom face of the CPV module 504 (see bottom face 922 in FIG. 9)is in direct thermal contact with the top face 606 of the cooling block604. However, the cooling block 604 may be alternatively shaped suchthat only a part of the bottom face of the CPV module 504 is in directthermal contact with the top face 606.

Cold heat transfer fluid 712 flows through an input tube 612 in the +zdirection to enter the cooling block 604 via an inlet 802 located on abottom face 804 of the cooling block 604. For clarity, the input tube612 is only shown in FIG. 7. Inside the cooling block 604, the heattransfer fluid travels along a serpentine path to absorb heat from thePV cells 306 through the top face 606 (see serpentine path 1210 in FIG.12B). After absorbing waste heat 312 from the PV cells 306, warm heattransfer fluid 714 exits the cooling block 604 via an outlet 806 andenters the helical tube 602 via a rigid transfer tube 614. Afterdirectly absorbing non-centered light 432 illuminating the helical tube602, hot heat transfer fluid 716 leaves the helical tube 602 via anoutput tube 608. In embodiments, the input tube 612 is a rigid tube thatmechanically supports all of the CPV module 504 and thermal module 502.

FIGS. 5-8 show the helical tube 602 as a square helix with approximatelyfifteen turns. However, the helical tube 602 may be alternativelyconfigured as a rectangular helix, a circular helix, or another type ofhelix. The helical tube 602 may also have a different number of turnsthan shown without departing from the scope hereof. In FIGS. 5-8, thehelical tube 602 is shown as tightly packed, i.e., each turn directlycontacts the one turn directly above it (when present), and the one turndirectly below it (when present). Tight packing advantageously increasesthe surface area with which non-centered light 432 and indirect light508 are captured by eliminating gaps between the turns. To enhance thestructural rigidity of the helical tube 602, adjacent turns may bebrazed or welded together, thereby filling gaps between the turns thatmay otherwise exist. However, the helical tube 602 may be alternativelyshaped with gaps between neighboring turns, again without departing fromthe scope hereof.

To enhance direct absorption of concentrated light 108, the helical tube602 may be coated with a black finish. For example, the helical tube 602may be coated with a highly absorptive high-temperature black solarpaint, such as Pyromark 2500, or a nanomaterial-based black coating.Alternatively, the outside surfaces of the helical tube 602 may besurface-treated to be black (e.g., anodization) or otherwise absorptive.

The helical tube 602 may be formed from a single metal tube bent into ahelical shape. For example, the metal may be aluminum, copper, or brass,which have high thermal conductivities, are commercially available astubes, and can welded or brazed using known techniques. However, thehelical tube 602 may be formed from a different type of metal (e.g.,stainless steel) without departing from the scope hereof. As shown inFIGS. 7 and 8, the output tube 608 may be a portion of the helical tube602 bent in the −z direction.

In an embodiment, and as shown in FIG. 7, a topmost turn 610 of thehelical tube 602 is positioned below the top face 606 by an offset 715.Subsequent turns of the helical tube 602 continue in the −z direction.The offset 715 may be selected to prevent non-centered light 432 fromdirectly illuminating side walls 618 of the cooling block 604, whichwould inadvertently heat the cooling block 604. For example, the offset715 may be equal to, or less than, a diameter of the helical tube 602.Alternatively, the offset 715 may be slightly negative, i.e., thetopmost turn 610 may be positioned slightly above the top face 606provided that it does not block concentrated light 108 from reaching thePV cells 308. Alternatively, the offset 715 may be larger than shown inFIG. 7 to minimize thermal conduction of heat from the helical tube 602into the cooling block 604, which would inadvertently heat the CPVmodule 504.

In FIGS. 5-8, each turn of the helical tube 602 encloses an area (in thex-y plane) slightly larger than that of the cooling block 604 such thatthe topmost turn 610 can block the side walls 618 of the cooling block604 from concentrated light 108. In this arrangement, the helical tube602 is positioned such that a gap 713 between the topmost turn 610 andthe side walls 618 is small (e.g., less than a diameter of the helicaltube 602). One or more thermal insulation pieces 710 with low thermalconductivity may be inserted between the cooling block 604 and helicaltube 602 to prevent the cooling block 604 and helical tube 602 fromcontacting each other, which would result in a thermal short. Thehelical tube 602 may be shaped to enclose a larger area so that the gap713 is larger than shown. A larger gap 713 may also help reduce thermalconduction of heat from the topmost turn 610 into the cooling block 604.However, a larger gap 713 may also result in more concentrated light 108illuminating the side walls 618. Furthermore, a larger gap 713 increasesthe thickness of the thermal insulation pieces 710 and therefore theamount of concentrated light 108 directly absorbed by these thermalinsulation pieces 710. This absorbed light is wasted since the resultingheat cannot be efficiency coupled to the heat transfer fluid.

FIG. 9 is a side cut-away view of the CPV module 504 in regions where aPV cell 306 is present. The layers in FIG. 9 are not drawn to scale.Concentrated light 108 propagating in the −z direction is transmittedthrough a superstrate 904 and an encapsulant 906 to reach the PV cell306. The superstrate 904 and encapsulant 906 protect the CPV module 504from external environmental conditions such as moisture and debris. Inembodiments, the superstrate 904 and encapsulant 906 are opticallytransparent, thermally shock resistant, mechanically rigid, and stableat the operating temperature of the CPV module 504.

In the example of FIG. 9, the PV cell 306 is a three junction cell witha first subcell 910(1), a second subcell 910(2), and a third subcell910(3). The PV cell 306 is electrically connected to an electricallyconductive backing sheet 914 using electrical adhesive 912 or solder.The backing sheet 914 is rigidly affixed to heat transfer plate 920 withthermal adhesive 916. A bottom face 922 of the heat transfer plate 920directly contacts the top face 606 of the cooling block 604. Bus bars923 on top of the first subcell 910(1) act as a cathode, while thebacking sheet 914 acts as an anode. While the example of FIG. 9 througha three-junction PV cell 306, the CPV module 504 may be configured withPV cells 306 have a different number of layers. For example, each PV 306may have only one or two subcells 910, or four or more subcells 910.Each PV cell 306 may have an anti-reflection coating on top of the firstsubcell 910(1) (not shown in FIG. 9).

The superstrate 904 may be made of crystalline quartz, fused silica,sapphire, glass, or another type of rigid optically transparentmaterial. The superstrate 904 may also have an anti-reflection coatingon one or both of its faces. The encapsulant 906 may be a siliconeelastomer using polydimethylsiloxane (PDMS), such as Dow Corning Sylgard184. The electrically conductive backing sheet 914 may be sheet or foilmade of silver or another type of metal with high electricalconductivity. The heat transfer plate 920 may be made of alumina, whichis an electrical insulator with a relatively high thermal conductivity.The heat transfer plate 920 may be alternatively made from another typeof ceramic, a different class of materials, or a combination ofelectrically insulating with having high thermal conductivity. In someembodiments, the backing sheet 914 is metal directly deposited (e.g.,via electroplating) onto the heat transfer plate 920. These embodimentsadvantageously avoid the need for thermal adhesive 916.

FIG. 10 is a top view of the CPV module 504 showing the layout of the PVcells 306. In this example, there are sixteen PV cells 306 arranged in atwo-dimensional array of four rows and four columns. The PV cells 306are grouped in four sets (also referred to as “quadrants”) and each ofthe PV cells 306 in FIG. 10 is labeled with the number of the set towhich it belongs. Each set also includes one half-sized PV cell 1004whose active area is approximately one-half of that of the other“full-sized” PV cells 306. The four half-size PV cells 1004 are sizedand positioned to create a shadow region 1010 within which the solarflux is minimal due to blockage by the support arm 112 (see FIGS. 1 and2).

All of the PV cells 306 within each set are electrically connected inparallel, and the sets are connected in series. More specifically, afirst wire 1002(1) connects to bus bars 923 on the front faces of thefour PV cells 306 in the set “1”, a second wire 1002(2) connects to thebus bars 923 on the front faces of the four PV cells 306 in the set “2”,and so on. Thus, the wires 1002 connect cathodes of the PV cells 306.Similar wires connecting the anodes of the PV cells 306 are not shown inFIG. 10. The anodes of the fourth set of PV cells 306 are connected to afirst electrical terminal (labeled “POS” in FIG. 10), and the first wire1002(1) connects the cathodes of the first set of PV cells 306 to asecond electrical terminal (labeled “NEG” in FIG. 10). The wires 1002may be made of silver or another metal with high electricalconductivity.

The PV cells 306 may have a different arrangement than shown in FIG. 10without departing from the scope hereof. For example, the PV cells 306may form a two-dimensional array with more than four rows or less thanfour rows, more than four columns or less than four columns, or acombination thereof. In FIG. 10, most of the PV cells 306 are spacedfrom their nearest neighbors by a gap of 1 mm. However, the PV cells 306may be spaced such that the gap has different value. The PV cells 306may also be wired in a different configuration, and therefore may begrouped into more than four sets or less than four sets. The number andpositions of the half-sized PV cells 1004 may also be changed to varythe size and location of the shadow region 1010 (e.g., to overlap theshadow caused by the support arm 112).

Design Methodology

CPV Module—To build a prototype of the hybrid receiver 110, a prototypeof the CPV module 504 was constructed using the geometry shown in FIGS.5-10. The superstrate 904 was made from GE 124 fused quartz, and theencapsulant 906 was Dow Corning Sylgard 184 silicone. The anodes of thePV cells 306 were adhered to a silver backing sheet 914 using SnPdelectrical solder as the electrical adhesive 912. Thermal adhesive 916(Cotronic Duralco 128) was applied between the silver backing sheet 914and the heat transfer plate 920 to increase thermal conductivitytherebetween. The heat transfer plate 920 was made of alumina because itis an electrical insulator with a high thermal conductivity of 25 W/m·Kat room temperature. This choice of material allows waste heat 312 fromthe PV cells 306 to be efficiently conducted to the cooling block 604without electrical shorts.

FIG. 11 shows exemplary current-voltage curves measured with atriple-junction PV cell 306 used with the prototype CPV module 504. Thedata in FIG. 11 was measured for concentrations of 1 sun and 492 suns ata temperature of 25° C. The triple-junction PV cells 306 were fabricatedwith n-on-p structure by SolAero Technologies, and built on germaniumsubstrates. The photovoltaic absorber materials of the triple-junctionPV cells 306 are InGaP (1.86 eV), InGaAs (1.40 eV), and Ge (0.67 eV). Ananti-reflection coating was deposited on the PV cells 306 to lowerreflectance between 0.3 μm and 1.8 μm. The V_(oc) is 2.52 V for 1 sunand 3.21 V for 492 suns, while the I_(sc) increases from 12.1 mA to 6.9A due to the highly concentrated incident light. The ratio of J_(sc) tothe light intensity (J_(sc)/intensity) is similar for both 1 sun (0.145A/W) and 492 suns (0.148 A/W). The power conversion efficiency η reaches37.8% for the AM1.5D spectrum under 492 suns.

The prototype CPV module 504 had a size of 47 mm×47 mm, within whichtwelve full-sized PV cells 306 and four half-sized cells 1004 wereplaced as shown in FIG. 10. Each of the full-sized PV cells 306 haddimensions of 9.5 mm×9.75 mm. Gaps of 1 mm between neighboring PV cell306 provide space for wiring and placing thermocouples. As shown in FIG.10, there are no PV cells 306 in the shadow area 1010 due to the shadowcast by the support arm 112 of FIGS. 1 and 2. The PV cells 306 weredivided into four quadrants with three full-sized PV cells 306 and onehalf-sized PV cell 1004 in each quadrant connected in parallel. The fourquadrants were wired in series, leaving the anode of quadrant 4 and thecathode of quadrant 1 as electrodes to extract electrical power from theprototype. With this wiring strategy, a 4-four increase in voltage and3.5-fold increase in current is achieved.

Thermal Module—A prototype of the thermal module 502 was designed toprioritize simple and low-cost assembly, low energy loss, strongmechanical rigidity, and long lifetime. A prototype of the cooling block604 was fabricated from two plates sealed together with a gasket.Internally, a 500 micron-deep serpentine path was machined into one ofthe plates, covering most cell areas. The size of the prototype coolingblock 604 is similar to the prototype CPV module 504, and includes anotch 616 (see FIG. 6) for electrical wires. The prototype cooling block604 was fabricated from 6061-T6 aluminum.

A prototype of the helical tube 602 was formed from 3003 annealedaluminum tubing with an outside diameter of 3.17 mm and a wall thicknessof 0.64 mm. This tubing was hand-bent into a square helix, as shown inFIGS. 5-8. Aluminum brazing was used to fill gaps between adjacentturns. This brazing was performed on two opposing inner faces of theprototype helical tube 602 to increase structural integrity andmechanical robustness. Coated in Pyromark 2500, a highly absorptiveblack solar paint, the prototype helical tube 602 can absorb incidentconcentrated light with an absorptance of 95.0%. Several thermalinsulation pieces 710 (Zircar RSLE-57) with low thermal conductivity of0.55 W/m·K at 200° C. were placed between the prototype cooling block604 and the prototype helical tube 602 to prevent the prototype CPVmodule 504 from being heated by the high temperatures of the prototypehelical tube 602.

Pipes with an inner diameter of 1.9 mm and an outer diameter of 3.2 mmwere connected between an inlet reservoir (not shown) and the inlet 802of the prototype cooling block 604, and between the output tube 608 andan outlet reservoir (not shown). These pipes also serve as the exclusivestructural supports for the entire hybrid receiver 110. Due to thelength and small cross-section area of these pipes, thermal conductionbetween high-temperature components of the thermal module 502 and thepipes is decreased, minimizing energy loss. Thermal insulation was alsowrapped around the outlet pipe to further minimize thermal loss.

The Wind Shroud—A square-shaped prototype of the shroud 506 wasconstructed of highly reflective (R>95%) optical-grade anodized aluminumsheet (Alanod MIRO 4400 GP). It acts as a secondary reflector toredirect scattered solar radiation and emitted thermal radiation backtoward the helical tube 602. It also reduces thermal convection lossfrom the thermal module 502 to the environment due to wind, which canotherwise be a prominent loss mechanism when the exterior surfacetemperature of the helical tube 602 exceeds 200° C.

Modeling

Electrical Model—The performance of each PV cell 306 was numericallymodeled according to the incident power under concentrated fluxirradiation. With these numbers, the electrical output of the CPV module504 was predicted based on the physical circuit configuration andKirchhoff's laws. The solar input flux to the prototype CPV module 504was assumed to be inhomogeneous, being calculated directly from fluxmaps at the plane of the PV cells 306. Power losses due to voltagemismatches inside each quadrant, current mismatches among the fourquadrants, increased series resistance due to cell wiring, and shadowingfrom the silver wires were taken into account in this model.

Thermal Model—Finite element method (FEM) modeling with COMSOL was usedto simulate the thermal performance of the cooling block 604 and thehelical tube 602. In the cooling-block model, the twelve full-sized PVcells 306 and four half-sized PV cells 1004 were treated as heatsources. The heat power was calculated using the incident power on theCPV module 504, power loss due to optical reflection, and the efficiencyof the CPV module 504 determined from the electrical model. Conditionslike inlet temperature and mass flow rate were set to expected valuesfor field operation.

FIG. 12 shows exemplary temperature contour plots for the prototype CPVmodule 504 (see FIG. 12A), cooling block 604 (see FIG. 12B), and helicaltube 602 (see FIG. 12C). The contour plots in FIG. 12 were obtain fromthe thermal model described above. The total power from the PV cells 306as waste heat 312 was set to 258.4 W, which is the thermal load when thehybrid receiver is positioned 60 mm inboard from the focal plane 442 ofthe dish 102 (average concentration on the cells of 345 suns). For theheat transfer fluid, water was selected with a mass flow rate of 2 g/sand an initial temperature of 20° C. The simulation predicted a maximumtemperature of 62.2° C. for the PV cells 306, and an outlet temperatureof 49.3° C.

In the model of the prototype helical tube 602, the solar irradiationpower of 1634.5 W was distributed uniformly on the outer surface. Theblack solar paint coating had a power emissivity of 0.92. Again, waterwith an initial temperature of 20° C. and mass flow rate was 2 g/s wasused as the heat transfer fluid. Considering that the output temperatureis expected to be high with non-negligible power loss, thermal radiationand convection losses were included for accuracy. As shown in FIG. 12C,the predicted temperature near the output tube 608 reaches 210.7° C.Radiation loss and convection loss together contribute 5% of the inputpower, which will be used for future performance evaluation.

In-Lab Characterization

Electrical Characterization—FIG. 13 shows exemplary current-voltagecurves measured with a “bare” triple-junction PV cell 306 and each ofthe four quadrants of the prototype CPV module 504. To obtain thesecurves, each of four quadrants was individually assembled and testedunder one-sun illumination provided by a continuous, multi-zone solarsimulator (Unisim, TS-Space System), with a total intensity of 0.09W/cm². The bare SolAero triple-junction PV cell 306 was testedsimilarly. The table in FIG. 13 lists the physical parameters for theone-sun tests. Since each quadrant contains 3.5 cells in parallel, a3.5-fold increase in I_(sc) was expected for each of the quadrants, ascompared to the bare PV cell 306. However, due to the shadowing fromsilver wires 1002, only a 3.2-fold increase in I_(sc) was observed. Allfour quadrants show a decreased V_(oc), which results from the seriesresistance of the silver wires 1002.

Thermal Characterization—FIG. 14 shows an exemplary setup 1400 andresults 1402 for tests of the prototype cooling block 604. As shown inthe setup 1400, heat-transfer putty was used to affix a strip heater1404 to the back side of an intermediate transitionary aluminum piece1408. A front side 1410 of the aluminum piece 1408 acts as the bottomface 922 of the CPV module 504 (see FIG. 9) and is placed in directcontact with the top face 606 of the prototype cooling block 604. Seventhermocouples (TCs) were positioned at various interfaces, as well as inthe inlet (TC1) and outlet (TC2) of the cooling block 604. The positionsof the seven TCs are identified in FIG. 14 by numbers 1 through 7. Thesetup 1400 was then wrapped tightly with mineral wool insulation toreduce heat loss. Room-temperature water flowed through the coolingblock 604 and the strip heater 1404 was set to output 57 W of heatpower. Water from the outlet was weighed every 60 seconds to ensureadherence to the desired mass flow rate. The TC temperatures weredisplayed and recorded on a computer that was connected to aneight-channel datalogger. The results 1402 show measured TC temperaturesas a function of elapsed time during the test. Tests were conducted formass flow rates of 1 g/s, 2 g/s, and 3 g/s. The results 1402 are for thetest at 2 g/s.

FIG. 15 shows an exemplary setup 1500 and results 1502 for tests of theprototype helical tube 602. Thermally conductive epoxy adhesive (3MTC-2707) was used to affix seven K-type thermocouples (TCs) to thehelical tube 602 at the locations labeled 1 through 7 in FIG. 15. Asilicone heating pad was then wrapped around the helical tube 602, againusing heat transfer putty to ensure reliable thermal contact. Thermalinsulation was set up with mineral wool to avoid energy losses.Room-temperature water flowed through the helical tube 602 at flow ratesbetween 1 g/s and 3 g/s while the silicone heating pad applied thermalenergy (230 W, 113 W, 59 W) to the helical tube 602. The results 1502show measured TC temperatures as a function of time with water flowingat 3 g/s mass flow rate and an input power of 230 W. As expected, thetemperature increases along the flow direction (top to bottom).Additional tests were conducted with different power inputs and massflow rates.

FIG. 16 shows exemplary plots of a convective heat transfer coefficienth as a function of mass flow rate for (A) the cooling block 604, and (B)the helical tube 602. The experimental values of h were obtained fromthe geometrical parameters of the fluid channel and the measuredtemperatures. The modeled values of h were calculated from results ofthe COMSOL simulation with the same boundary conditions. Theexperimental and modeled values of h agree, showing the expected linearrelationship with the mass flow rate. These results verify that thewater used as the heat transfer fluid is laminar in this region. Thehelical tube 602 performs similarly to the cooling block 604, but withsmaller values of h, in agreement with the design (i.e., the smaller thecooling channels, the larger the values of h).

Fluid Flow Characterization—FIG. 17 shows an exemplary plot of pressuredrop as a function of mass-flow rate. After mounting the prototypecooling block 604 and helical tube 602 together to form a prototypethermal module 502, gravimetric analysis was used to characterize therelationship between mass flow rates and applied pressures to the waterin the prototype thermal module 502. The thermal module 502 waspositioned approximately six feet above a water pump to simulate thehydraulic head experienced when installed on the solar tracker. Using apressure regulator, the water pressure was set to a discrete integervalue. All outflow was collected in a reservoir and weighed every 60seconds for 10 minutes. The average weight for all intervals was thencalculated and converted to mass. This process was repeated foradditional increasing integer pressure values, with the complete rangeof tested pressure values stretching from 3 to 25 psi. The data pointsin FIG. 17 are the measured values of the pressure drop, which arewell-fit to a quadratic line, representing non-laminar flow in theintegrated thermal module. This may result from stagnant water inchannel corners in the cooling block, or turbulent flow in some tubebending areas and piping connection ports.

Submerged Static Pressurization Test—To confirm the ability of theprototype thermal module 502 to contain the pressure necessary toconfine the water to the liquid phase while operating at outputtemperatures exceeding 200° C., the outlet 806 was sealed and the inlet802 was connected to pressurized argon gas. The tank valve was closedafter pressurizing the thermal module 502 to 400 psi. The thermal module502 was submerged in water for 5 minutes to check for gas leaks. Nobubbles or decrease in pressure was detected.

Outdoor Performance Prediction

Flux maps were measured and analyzed to identify the power distributionon both the prototype CPV module 504 and the prototype thermal module502. According to the electrical model, the electric power output andthe thermal load can be calculated. The thermal load from each PV cell306 was then imported to the COMSOL cooling-block model to predict thePV cell temperature and water temperature when it exits the coolingblock 604. After setting the power incident onto the helical tube 602 asthe input power and the exit water temperature from the cooling block604 as the inlet temperature in the COMSOL thermal-coil model, the finaloutput water temperature was predicted.

Solar Flux Analysis—FIG. 18 shows an exemplary flux map measured for adish and tracker that may be used to test the prototype hybrid receiver110. The flux map was measured at a position 63.5 mm from the focalplane in the direction toward the dish (i.e., inboard). As shown in FIG.18, the direct normal irradiance (DNI) was only 800 W/m² and the totalpower was 1973 W, as measured with a pyrheliometer mounted on thetracker. The flux distribution in the flux map has been scaled up to anormal DNI of 900 W/m², with a total power of 2220 W from the dish, forthe following models. Additional flux maps at 45, 50, 55, 60, 65, 70,75, and 80 mm were also obtained by the triangular projection method,assuming the rays converge towards the focal point.

MATLAB code was used to analyze the flux maps using the DNI number andthe known diameter of the pyrheliometer. With the flux density and powerfor each pixel in the image calculated from the MATLAB code, theincident power on the prototype CPV module 504 and each PV cell 306 wasobtained by integrating the power flux over the corresponding areas. InFIG. 18, the CPV module 504 is not fully overlaid on the flux map,leaving a large fraction of the incident power to reach the helical tube602.

FIG. 19 shows an exemplary plot of solar power incident on the CPVmodule 504 and the helical tube 602 as a function of cell-plane distanceinboard from the focal plane 442 of the dish 102. At 45 mm, the incidentlight is almost split equally between the CPV module 504 and the thermalmodule 502. Along the direction closer to the dish 102 (i.e., away fromthe focal plane 442), the power on the helical tube 602 increases sincethe flux area increases, with less sunlight being incident on the CPVmodule 504.

Predicted Outdoor Performance—FIG. 20 shows exemplary plots of predictedperformance of the prototype hybrid receiver 110. The flux density andpower on each PV cell 306 from the flux map analysis was inputted to theelectrical model discussed above. The model predicts electric power fromthe CPV module 504. This process was conducted multiple times for eachposition of the CPV module 504 relative to the focal plane 442 of theconcentrator dish 102. FIG. 20A shows the predicted electric power as afunction of distance from the focal plane 442. It shows the same trendas the helical tube 602 in FIG. 19. The efficiency of the CPV module 504is between 29.8% and 32.1%. Voltage and current mismatch is the largestsource of lost power, corresponding to almost 3% of the power incidenton the PV cells 306.

PV-cell temperature was investigated using the above-mentioned thermalmodel. It is required that the temperatures of the PV cells 306 neverexceed the upper limit operating temperature of the CPV module 504,usually around 110° C. FIG. 20B shows the predicted maximum celltemperature for mass flow rates of 1 g/s, 2 g/s, and 3 g/s. The PV cells306 are cooler at higher mass flow rate, as expected, because theconvective heat transfer coefficient increases linearly with themass-flow rate, as described above. All points except for that at 45 mmand 1 g/s are below 110° C.

The output temperature from the thermal module 502 was also investigatedusing the above-mentioned thermal model. For wider applications ofprocess heat in industry or commercial use, an output temperature largerthan 120° C. may be preferred. FIG. 20C shows the output temperature asa function of the distance from the focal plane for three mass flowrates. The final output temperature is largely impacted by the mass flowrate, not position relative to the focal plane. The water absorbs bothwaste heat 312 from the CPV module 504 and the solar power striking theouter surface of the helical tube 602. Even though more power isabsorbed by the helical tube 602 as the CPV module 504 is moved awayfrom the focal plane 442, the lower exit temperature of the coolingblock 604 (and corresponding input temperature to the helical tube 602)compensates the temperature difference among different cell operatingplanes. The operating parameters used in the thermal model all result inan output temperature greater than 120° C. The model predicts a 94.6%efficiency of the helical tube 602 for an operating plane of 63.5 mm anda mass flow rate of 2 g/s.

FIG. 21 is a table listing predicted electrical and thermal energiesgenerated by the prototype hybrid receiver 110 when operating atdifferent distances between the CPV module 504 and the focal plane 442.The data in FIG. 21 was obtained assuming a mass flow rate of 2 g/s. Theelectrical energy and thermal energy are adjustable by moving the hybridreceiver 110 closer to or further away from the focal plane. The CPVTefficiency is obtained by adding the energy fractions of electric powerand thermal power. The CPVT efficiency is larger when the hybridreceiver is positioned farther from the focal plane 442, as expectedsince the thermal absorption efficiency in the thermal module 502increases faster than the electrical conversion efficiency in the CPVmodule 504 decreases.

Combination of Features

Features described above as well as those claimed below may be combinedin various ways without departing from the scope hereof. The followingexamples illustrate possible, non-limiting combinations of features andembodiments described above. It should be clear that other changes andmodifications may be made to the present embodiments without departingfrom the spirit and scope of this invention:

(A1) A hybrid receiver may include a concentrator photovoltaic modulehaving a heat transfer plate and an array of photovoltaic cellsthermally coupled to the heat transfer plate. The hybrid receiver mayinclude a thermal module having a cooling block forming an internal paththrough which heat transfer fluid can flow. A bottom face of the heattransfer plate may be in direct thermal contact with a top face of thecooling block. The thermal module may have a helical tube surroundingthe cooling block and connected to the cooling block such that the heattransfer fluid flows through the helical tube after flowing through thecooling block.

(A2) In the hybrid receiver denoted (A1), the hybrid receiver mayinclude a reflective shroud surrounding the concentrator photovoltaicmodule and the thermal module. The reflective shroud may be shaped toreflect thermal radiation and scattered light from the thermal moduleonto the helical tube.

(A3) In the hybrid receiver denoted (A2), the reflective shroud may bemade of aluminum.

(A4) In any one of the hybrid receivers denoted (A1) to (A3), the hybridreceiver may include an inlet pipe connected to a rear face of thecooling block, and an outlet pipe connected to an output port of thehelical tube. The inlet pipe may mechanically support the thermal moduleand the concentrator photovoltaic module.

(A5) In any one of the hybrid receivers denoted (A1) to (A4), the hybridreceiver may include a plurality of thermally insulating spacerspositioned between the cooling block and the helical tube.

(A6) In any one of the hybrid receivers denoted (A1) to (A5), the hybridreceiver may include an absorptive coating on the helical tube.

(A7) In any one of the hybrid receivers denoted (A1) to (A6), the heattransfer plate may be made of an electrically insulating material.

(A8) In the hybrid receiver denoted (A7), the electrically insulatingmaterial may be alumina.

(A9) In any one of the hybrid receivers denoted (A1) to (A8), all of thebottom face of the heat transfer plate may be in direct thermal contactwith the top face of the cooling block.

(A10) In any one of the hybrid receivers denoted (A1) to (A9), thehelical tube may form either a square helix or a circular helix.

(A11) In any one of the hybrid receivers denoted (A1) to (A10), each ofthe array of photovoltaic cells may be a III-V multi junction cell.

(A12) In any one of the hybrid receivers denoted (A1) to (A11), theconcentrator photovoltaic module may include an electrically conductivebacking sheet, electrical adhesive bonding a rear face of each of thearray of photovoltaic cells to the backing sheet, and thermal adhesivebonding the backing sheet to a top face of the heat transfer plate.

(A13) In the hybrid receiver denoted (A12), the backing sheet may bemade of silver.

(A14) In any one of the hybrid receivers denoted (A1) to (A13), thehybrid receiver may include a superstrate window positioned over thearray of photovoltaic cells, and an encapsulant joining a front face ofeach of the array of photovoltaic cells with a bottom face of thesuperstrate window.

(A15) In any one of the hybrid receivers denoted (A1) to (A14), theinternal path may be sized such that heat transfer fluid flows throughthe internal path as a microfluid.

(A16) In any one of the hybrid receivers denoted (A1) to (A15), theinternal path may have a depth of 500 μm or less.

(B1) A concentrator photovoltaic-thermal power system may include anyone of the hybrid receivers denoted (A1) to (A16), a concentratormirror, and a solar tracker with a support arm affixed to the hybridreceiver and positioning the hybrid receiver such that the concentratorphotovoltaic module faces the concentrator mirror.

(B2) In the concentrator photovoltaic-thermal power system denoted (B1),the support arm may position the hybrid receiver such that theconcentrator photovoltaic module is between the concentrator mirror anda focal plane of the concentrator mirror.

(C1) A method for concentrated photovoltaic-thermal power generation mayinclude converting a first portion of concentrated sunlight intoelectrical power when the first portion of concentrated sunlightilluminates an array of photovoltaic cells, thermally coupling heatgenerated by the photovoltaic cells into a heat transfer plate, coolingthe heat transfer plate by flowing heat transfer fluid through aninternal path of a cooling block in direct thermal contact with the heattransfer plate, and flowing the heat transfer fluid through the helicaltube to absorb thermal energy from a second portion of concentratedsunlight illuminating the helical tube.

(C2) In the method denoted (C1), the method may include reflectingthermal radiation and scattered light from the thermal module onto thehelical tube.

(C3) In the method denoted (C2), said reflecting may be performed with areflective shroud surrounding the helical tube, cooling block, heattransfer plate, and array of photovoltaic cells.

(C4) In the method denoted (C3), the method may include blocking windfrom the helical tube and cooling block with the reflective shroud.

(C5) In any one of the methods denoted (C1) to (C4), the method mayinclude supplying the heat transfer fluid to the cooling block via aninlet pipe connected to a rear face of the cooling block, and receivingthe heat transfer fluid from the helical tube with an outlet pipe. Themethod may also include mechanically supporting the helical tube,cooling block, heat transfer plate, and array of photovoltaic cells withthe inlet pipe.

(C6) In any one of the methods denoted (C1) to (C5), the method mayinclude thermally isolating the cooling block from the helical tube witha plurality of thermally insulating spacers.

(C7) In any one of the methods denoted (C1) to (C6), the method mayinclude positioning the array of photovoltaic cells, heat transferplate, cooling block, and helical tube to adjust a ratio of electricalpower generated by the array of photovoltaic cells and thermal poweroutputted by the heat transfer fluid.

(C8) In any one of the methods denoted (C1) to (C7), the heat transferfluid may flow through the helical coil after the cooling block suchthat an average temperature of the heat transfer fluid, when exiting thehelical coil, is greater than a maximum temperature of the photovoltaiccells.

Changes may be made in the above methods and systems without departingfrom the scope hereof. It should thus be noted that the matter containedin the above description or shown in the accompanying drawings should beinterpreted as illustrative and not in a limiting sense. The followingclaims are intended to cover all generic and specific features describedherein, as well as all statements of the scope of the present method andsystem, which, as a matter of language, might be said to falltherebetween.

What is claimed is:
 1. A hybrid receiver, comprising: a concentratorphotovoltaic module having a heat transfer plate and an array ofphotovoltaic cells thermally coupled to the heat transfer plate; and athermal module having: (i) a cooling block forming an internal paththrough which heat transfer fluid can flow, a bottom face of the heattransfer plate being in direct thermal contact with a top face of thecooling block; and (ii) a helical tube surrounding the cooling block andconnected to the cooling block such that the heat transfer fluid flowsthrough the helical tube after flowing through the cooling block.
 2. Thehybrid receiver of claim 1, further comprising a reflective shroudsurrounding the concentrator photovoltaic module and the thermal module,and shaped to reflect thermal radiation and scattered light from thethermal module onto the helical tube.
 3. The hybrid receiver of claim 2,the reflective shroud being made of aluminum.
 4. The hybrid receiver ofclaim 1, further comprising an inlet pipe connected to a rear face ofthe cooling block, and an outlet pipe connected to an output port of thehelical tube; the inlet pipe mechanically supporting the thermal moduleand the concentrator photovoltaic module.
 5. The hybrid receiver ofclaim 1, further comprising a plurality of thermally insulating spacerspositioned between the cooling block and the helical tube.
 6. The hybridreceiver of claim 1, further comprising an absorptive coating on thehelical tube.
 7. The hybrid receiver of claim 1, the heat transfer platebeing made of an electrically insulating material.
 8. The hybridreceiver of claim 7, the electrically insulating material being alumina.9. The hybrid receiver of claim 1, wherein all of the bottom face of theheat transfer plate is in direct thermal contact with the top face ofthe cooling block.
 10. The hybrid receiver of claim 1, the helical tubeforming either a square helix or a circular helix.
 11. The hybridreceiver of claim 1, wherein each of the array of photovoltaic cells isa III-V multi junction cell.
 12. The hybrid receiver of claim 1, theconcentrator photovoltaic module further comprising: an electricallyconductive backing sheet; electrical adhesive bonding a rear face ofeach of the array of photovoltaic cells to the backing sheet; andthermal adhesive bonding the backing sheet to a top face of the heattransfer plate.
 13. The hybrid receiver of claim 12, the backing sheetbeing made of silver.
 14. The hybrid receiver of claim 1, furthercomprising: a superstrate window positioned over the array ofphotovoltaic cells; and an encapsulant joining a front face of each ofthe array of photovoltaic cells with a bottom face of the superstratewindow.
 15. The hybrid receiver of claim 1, wherein the internal path issized such that heat transfer fluid flows through the internal path as amicrofluid.
 16. The hybrid receiver of claim 1, the internal path havinga depth of 500 μm or less.
 17. A concentrator photovoltaic-thermal powersystem, comprising: the hybrid receiver of claim 1; a concentratormirror; and a solar tracker with a support arm affixed to the hybridreceiver and positioning the hybrid receiver such that the concentratorphotovoltaic module faces the concentrator mirror.
 18. The concentratorphotovoltaic-thermal power system of claim 17, wherein the support armpositions the hybrid receiver such that the concentrator photovoltaicmodule is between the concentrator mirror and a focal plane of theconcentrator mirror.
 19. A method for concentrated photovoltaic-thermalpower generation, comprising: converting a first portion of concentratedsunlight into electrical power when the first portion of concentratedsunlight illuminates an array of photovoltaic cells; thermally couplingheat generated by the photovoltaic cells into a heat transfer plate;cooling the heat transfer plate by flowing heat transfer fluid throughan internal path of a cooling block in direct thermal contact with theheat transfer plate; and flowing the heat transfer fluid through thehelical tube to absorb thermal energy from a second portion ofconcentrated sunlight illuminating the helical tube.
 20. The method ofclaim 19, further comprising reflecting thermal radiation and scatteredlight from the thermal module onto the helical tube.
 21. The method ofclaim 20, wherein said reflecting is performed with a reflective shroudsurrounding the helical tube, cooling block, heat transfer plate, andarray of photovoltaic cells.
 22. The method of claim 21, furthercomprising blocking wind from the helical tube and cooling block withthe reflective shroud.
 23. The method of claim 19, further comprising:supplying the heat transfer fluid to the cooling block via an inlet pipeconnected to a rear face of the cooling block; receiving the heattransfer fluid from the helical tube with an outlet pipe; andmechanically supporting the helical tube, cooling block, heat transferplate, and array of photovoltaic cells with the inlet pipe.
 24. Themethod of claim 19, further comprising thermally isolating the coolingblock from the helical tube with a plurality of thermally insulatingspacers.
 25. The method of claim 19, further comprising positioning thearray of photovoltaic cells, heat transfer plate, cooling block, andhelical tube to adjust a ratio of electrical power generated by thearray of photovoltaic cells and thermal power outputted by the heattransfer fluid.
 26. The method of claim 19, wherein the heat transferfluid flows through the helical coil after the cooling block such thatan average temperature of the heat transfer fluid, when exiting thehelical coil, is greater than a maximum temperature of the photovoltaiccells.